Allowing "Removal" of Schedule Layers

[stmcallister]

This allows for the "removal" of schedule layers. Removal is in quotes because the PagerDuty API doesn't allow for deleting of schedule layers--you can only end them. That is what this PR does is set the end value for a schedule layer when you remove the layer from your tf config. Also, added tests for setting the end for a schedule layer, as well as updated the TestAccPagerDutySchedule_Multi test to include updating a schedule by removing a layer.

Finally, removed the ForceNew on schedule layers. Now changes to schedule layers will not force a new creation of a schedule object. Which addresses #43

TF_ACC=1 go test -run "TestAccPagerDutySchedule*" ./pagerduty -v -timeout 120m
=== RUN   TestAccPagerDutySchedule_import
--- PASS: TestAccPagerDutySchedule_import (6.39s)
=== RUN   TestAccPagerDutySchedule_Basic
--- PASS: TestAccPagerDutySchedule_Basic (8.83s)
=== RUN   TestAccPagerDutyScheduleOverflow_Basic
--- PASS: TestAccPagerDutyScheduleOverflow_Basic (7.67s)
=== RUN   TestAccPagerDutySchedule_BasicWeek
--- PASS: TestAccPagerDutySchedule_BasicWeek (7.28s)
=== RUN   TestAccPagerDutySchedule_Multi
--- PASS: TestAccPagerDutySchedule_Multi (12.10s)
PASS
ok  	github.com/terraform-providers/terraform-provider-pagerduty/pagerduty	42.738s

[heimweh]

Nice! 🙌
This LGTM 🙂👍